The present invention relates to a power supply unit for use in an electric discharge machine (EDM), and more particularly to an EDM power supply unit capable of changing a pulse width of a discharge current depending upon fluctuation of the power supply voltage.
An electrical circuit arrangement of a prior art EDM is shown in block form in FIG. 1. The circuit includes a regulated D.C. power supply 101, a resistor 102 connected in series with the power supply 101, a capacitor 105 having one terminal connected to the resistor 102 and another terminal connected to the power supply 101, and a switching transistor 104 interposed between the power supply 101 and the resistor 102. A pulse generator 103 is connected to the switching transistor 104 for ON-OFF controlling the latter. A machining electrode 106 and a workpiece 107 disposed in confronting relation to each other are connected across the capacitor 105.
According to the above-described electrical arrangement, the transistor 104 is cyclically rendered on and off in response to pulse signals fed from the pulse generator 103 which signals are produced at a constant time interval. Discharge machining is carried out in accordance with a voltage developed across the capacitor 105 which voltage has been maintained at substantially constant due to the regulated power supply 101.
However, it has been required that the EDM have a large output capability. To comply with such a requirement, the regulated power supply needs to be large in capacity. However, such a power supply is large in size and expensive in cost.